1. Field of the Invention
The present invention relates to an aspherical lens and an optical instrument using the aspherical lens.
2. Related Background Art
Generally, when a aspherical lens is to be fabricated by means of glass molding, since the dimension of the aspherical lens to be molded is restricted by required precision thereof, degree of freedom upon fabricating an aspherical lens in particular a large one becomes narrow. Even if a large diameter aspherical lens can be fabricated, the fabricating cost becomes extremely high. Moreover, an aspherical lens is fabricated by means of a fine grinding or a polishing, in addition to the high fabrication cost, it has a structural difficulty in grinding a concave surface, so that it becomes crucial restriction on designing an aspherical surface.
Accordingly, a compound type aspherical lens constructed by forming an optical member made of a resin and the like with an aspherical surface on a spherically polished optical substrate such as a glass substrate has been proposed. An optical system in which such an aspherical lens is installed has been proposed in Japanese Patent Application Laid-Open No. 2000-227548. However, in this compound type aspherical lens, when the shape of the resin aspherical surface becomes largely different from that of the spherical glass substrate, the thickness of the resin becomes large, so that it becomes difficult to fabricate an aspherical surface with high precision. Moreover, there is a defect that variation in the shape excessively increases upon variation in environment such as temperature and humidity. Accordingly, it has been impossible to realize a compound type aspherical lens with a large diameter and a large resin thickness.
In such background, Japanese Patent Application Laid-Open No. 63-157103 has proposed a compound type aspherical lens that an aspherical surface with low precision is formed on a glass substrate by means of a fine grinding or a polishing and, on this aspherical surface, a resin aspherical surface having high precision and a substantially equal thickness is formed.
However, in the compound type aspherical surface disclosed in Japanese Patent Application Laid-Open No. 63-157103, in order to prevent the aspherical surface with low precision formed on the glass substrate from performing refraction effect, a glass material having low refractive index corresponding to refractive index of the resin is applied as the glass substrate. Accordingly, in this technology, since refractive index of the glass substrate is fixed to that of the resin, degree of freedom in design has been excessively restricted.
When a wide-angle lens or a super wide-angle lens is to be designed by using such a compound type aspherical lens as, in particular, a negative lens, since a glass material having refractive index and Abbe number corresponding to the resin material is applied, it becomes difficult to correct Petzval sum and lateral chromatic aberration. Accordingly, a compound type aspherical surface having difference in refractive indices between glass substrate and the resin material with forming an aspherical surface on the glass substrate has not been proposed.